The present invention relates to optical multichannel transmission via optical fiber communication network, and more particularly to Wavelength-Division Multiplexing (WDM) transmission systems and methods utilizing multi-line optical sources.
Modem Dense Wavelength Division Multiplexing (DWDM) systems utilize narrow-band receivers equipped with interference filters, fiber Bragg gratings, or Array Waveguide Gratings and stable narrow-band light sources such as Distributed FeedBack (DFB) or Distributed Bragg Resonator (DBR) semiconductor lasers. This transmitter/receiver technology allows for sending multiple information channels over one physical fiber line. Conventional WDM systems may comprise 80 or more information channels propagating in a single glass fiber within the spectral band of about 1550 nm corresponding to minimum optical loss, and separated in frequency domain by internationally accepted 50, 100 or 200 GHz (ITU grid). To provide reliable and stable multichannel transmission, the light sources and receivers/filters must have high spectral accuracy. The bandwidths of temperature stabilized DFB are typically several MHz. This very narrow spectral output and respective long coherence length significantly limit the next generation of WDM systems development. For example, the laser output power per WDM channel is limited by interference of mutually coherent electromagnetic fields propagating in adjacent WDM channels.
There are two types of processes causing coherent interference, or cross-talk. The first type relates to nonlinear interaction of adjacent WDM channels, the most pronounced effects of which are known as four-wave mixing and cross-phase modulation. Four-wave mixing is a process where interaction of three strong fields of different optical frequencies produces the fourth field having a frequency that falls, together with the other WDM channel frequencies, within a receiver bandwidth. The four-wave mixing product is detected together with the WDM signal and produces beating noise. Cross-phase modulation is another optical nonlinear process where the optical pulse is distorted by perturbation of the fiber index of refraction due to optical Kerr effect induced by optical pulses propagating in adjacent WDM channels.
Four-wave mixing and cross-phase modulation set fundamental limits for maximum optical power per WDM channel that can be launched into the fiber. These effect also determine minimum frequency separation between the WDM channels and the minimum fiber chromatic dispersion required to reduce the coherence length between electromagnetic fields of different frequencies [F. Forghieri, R. W. Tkach, and A. R. Chraplyvy, xe2x80x9cFiber nonlinearities and their impact on transmission systemsxe2x80x9d, Chapter 8 in Optical Fiber Communication Systems, Vol. IIIA, Ed. I. P. Kaminow and T. L. Koch, Academic Press, San Diego, 1997].
Another type of processes causing WDM transmission impairment arises from coherent cross-talk related to non-complete extinction of WDM signals from WDM system components such as add-drop multiplexers (ADMs) and optical cross-connects (OXCs). Even for a relatively large extinction ratio, for example, about xe2x88x9220 dB, the remains of the dropped traffic mixed with added traffic produce xe2x88x9210 dB amplitude noise if the cross-talk is coherent. If the cross-talk is incoherent, the respective noise is xcx9c10 dB less. The coherent cross-talk power penalty scales rapidly with the number of cross-talk terms beating at the receiver imposing severe limitations onto component specifications of ADMs and OXCs [E. L. Goldstein, et al., IEEE PTL, Vol. 7, p.93, 1995]. In the next generation WDM networks, with optical switching and wavelength routing functionalities introduced, the cross-talk terms will be generated at each add-drop and cross-connect node. Suppression of coherent cross-talk is an important requirement to these new networks design.
It is therefore the main object of the present invention is to reduce coherent cross-talk between WDM channels of WDM optical network and improve the quality of signal transmission.
Accordingly the present invention provides method and system of transmitting optical signals generated by multi-line optical source via multichannel optical network.
A plurality of multi-line sources generates a corresponding plurality of optical signals within designated spectral ranges. Each designated spectral range xcex94xcexa9 comprises N spectral lines, each said spectral line having a spectral width xcex94xcfx89, where xcex94xcfx89 less than  less than xcex94xcexa9/ Nxe2x88x921. Generated light is modulated within each WDM optical channel by a respective electrical signal for obtaining a respective modulated optical signal. A ratio of xcex94xcexa9/ Nxe2x88x921 defines a spectral separation between adjacent spectral lines, and this spectral separation exceeds a spectral bandwidth of the electrical signal for each optical channel. The modulated optical signals are propagating via respective WDM optical channels trough an optical fiber of the WDM optical network. Depending on the applications, a dispersion shifted fiber (DSF) or a standard single mode fiber (SMF) may be utilized. The propagated optical signals are detected and non-linear optical interaction of the propagated optical signals are suppressed.
According to another aspect of the present invention a multichannel WDM optical network comprises at least two WDM point-to-point systems interconnected by an optical cross-connect. This network comprises a plurality of multi-channel optical sources for generating a plurality of optical signals within designated spectral ranges corresponding to spectral ranges of a plurality of channels of the WDM optical network. Each designated spectral range comprises a plurality of spectral lines evenly separated therebetween. A spectral separation between adjacent spectral lines exceeds a spectral bandwidth of a signal transmitted via each WDM channel. Incorporation of this multi-line source to the WDM network allows for substantial suppression of coherent cross talk residue associated with optical cross connect.